Compression extrusion is a known method for pressurizing snow by a hydraulic operated piston in a cylinder to produce solid dry ice blocks or discs. For instance, U.S. Pat. No. 4,780,119 discloses a method and apparatus for producing dry ice solid discs from liquid carbon dioxide. Conventional pellitizers are shown in U.S. Pat. No. 4,617,064 wherein liquid carbon dioxide is introduced into a piston cylinder and is compressed into a snow-like solid. The snow is further compacted and forced under pressure from the piston to pass through a die having a large number of bores to produce solid dry ice particles. The pellets can be conveyed to a discharge nozzle adapted to spray discharge the pellets at high velocity and pressure to blast an object for cleaning purposes. The dry ice pellets projected by the nozzle under pressurized gas provide the desired solid particles impact on the object. The particles readily sublime from the solid carbon dioxide to vapor phase carbon dioxide and thereby avoid a clean up mess typical with sand particles. Blasting with dry ice pellets is further suggested in U.S. Pat. No. 4,038,786. Conventional prior art methods of dry ice blasting, however, ordinarily utilize random size dry ice particles or collect dry ice particles by size distribution for use in the spray nozzle blasting equipment. Random size pellets extruded in varying non-uniform sizes and lengths are unevenly propelled under pressure while blasting to provide uneven impact and non-uniform results with the target surface being cleaned.
It now has been found that improved particle blasting results can be achieved by using uniform size dry ice pellets having a constant length, a constant width or diameter, and a constant solid density, where the preferred pellets comprise uniform dimensions. Preferred cylindrical pellets for instance would uniformly have the diameter and a length dimensions identical with all particles having the same density or uniform weight per volume. In accordance with this invention a dry ice pellet extruder is controlled by preset computer signals to compress liquid carbon dioxide into a loose snow solid followed by compacting the snow into a solid dry ice block within the extruder at a predetermined density of a set weight per volume. The extruder then automatically extrudes solid ice particles at the predetermined density to a predetermined length in conjunction with the set dimension of the die openings. The uniform size and equal density dry ice particles can then be collected and conveyed to a pressurized spray nozzle device for particle blasting. The uniform density and uniform size dry ice pellets blast target surfaces at constant velocity and produce substantially improved blasting results. These and other advantages will become more apparent by referring to the drawings and the detailed description of the invention.